In vapour compression systems fluid medium, such as refrigerant, is circulated along a refrigerant path wherein the components of the vapour compression system are arranged. The fluid medium is compressed in a compressor. The compressed fluid medium is then fed to a condenser, where the compressed fluid medium is condensed, the fluid medium leaving the condenser thereby being substantially in a liquid state. The fluid medium is then fed to an expansion device, where it is expanded before entering an evaporator. In the evaporator the fluid medium is evaporated before once again entering the compressor, thereby completing the cycle.
As the fluid medium is evaporated in the evaporator, heat exchange takes place between the fluid medium and a secondary fluid flow across the evaporator, thereby cooling the fluid of the secondary fluid flow. This may be used for providing refrigeration to a closed volume, such as a room or a refrigeration entity, e.g. of the kind used in supermarkets. In the case that the difference between the temperature of the incoming secondary fluid flow and the desired outlet temperature is relatively large, it is necessary to control the operation of the vapour compression system in such a manner that the evaporator temperature, and thereby the pressure in the evaporator, is very low, in order to ensure a sufficiently high refrigeration capacity. This is undesirable, since it is very energy consuming, in particular because a relatively high amount of energy is consumed by the compressor in order to compress the low pressure fluid medium leaving the evaporator.
For instance, in the case that the vapour compression system is an air condition system, the fluid of the secondary fluid flow is air which is refrigerated, due to heat exchange with the fluid medium evaporating in the evaporator, in order to reduce the temperature inside an enclosure, such as a room. In some cases it may be required to reduce the temperature of air flowing across the evaporator from approximately 26° C. to approximately 10° C. in order to obtain a desired temperature of the enclosure. In this case the evaporator temperature must be maintained below 10° C.
U.S. Pat. No. 2,215,327 discloses an air condition system comprising an evaporator with two evaporator coils arranged fluidly in parallel in the refrigerant path. The evaporator coils are further arranged in series with respect to the path of the air circulated across the evaporator. One of the evaporator coils is maintained at a higher refrigerant pressure and surface temperature than the other evaporator coil. The evaporator coil with the higher surface temperature is used for lowering the temperature of the air passing over the evaporator, and the evaporator coil with the lower temperature is used for lowering the temperature of the air passing over the evaporator as well as for lowering the humidity of the air passing over the evaporator. In order to maintain the evaporator coils at different pressures, each evaporator coil is provided with a suction pressure control valve which controls the flow of refrigerant through the corresponding evaporator coil. The valves are of the same construction, but are adjusted to maintain different refrigerant pressures in the evaporator coils.
The suction pressure control valves are arranged fluidly between the evaporator coils and a common suction line being fluidly connected to the compressor. The suction pressure control valves reduce the pressure of the refrigerant leaving the evaporator coils, and the refrigerant pressure prevailing in the common suction line is therefore lower than the refrigerant pressure of the refrigerant leaving at least one of the evaporator coils. Accordingly, the energy consumed by the compressor in order to compress the refrigerant received via the common suction line is relatively high.
Furthermore, the system comprises expansion valves arranged fluidly in front of each of the evaporator coils, and the expansion valves are provided with thermostatic elements or bulbs respectively secured to the coils adjacent the outlets thereof. Accordingly, the expansion valves are operated independently of each other.